BAGHDAD — Militants from the Islamic State of Iraq and the Levant boasted on Twitter that they had executed 1,700 Iraqi government soldiers, posting gruesome photos to support their claim.

The authenticity of the photographs and the insurgents’ claim could not be verified, and Iraqi government officials initially cast doubt on whether such a mass execution took place. There were also no reports of large numbers of funerals in the Salahuddin province area, where the executions were said to have been conducted.

If the claim is true, it would be the worst mass atrocity in either Syria or Iraq in recent years, surpassing even the chemical weapons attacks in the Syrian suburbs of Damascus last year, which killed 1,400 people and were attributed to the Syrian government.

The latest attack, if proved, would also raise the specter of the war in Iraq turning genocidal, particularly because the insurgents boasted their victims were all Shiites. There were also fears it could usher in a series of reprisal killings of Shiites and Sunnis, like those seen in the Iraq War in 2005-07.

The office of the Shiites’ supreme spiritual leader, Grand Ayatollah Ali al-Husseini al-Sistani, issued Saturday night what amounted to a revision of the ayatollah’s call to arms Friday, apparently out of concern it was misinterpreted by many as a call for sectarian warfare.

The statement, billed as “clarifying the position on taking up arms,” implored Iraqis, “especially those living in mixed areas, to exert the highest level of self-restraint during this tumultuous period.”

The mass execution claim appeared on a Twitter feed previously used for Islamic State of Iraq and the Levant announcements, so whether or not the executions were genuine, the organization certainly intended to boast of them.

“We’re trying to verify the pics, and I am not convinced they are authentic,” said Erin Evers, the Human Rights Watch researcher in Iraq. “As far as ISIS claiming it has killed 1,700 people and publishing horrific photos to support that claim, it is unfortunately in keeping with their pattern of commission of atrocities, and obviously intended to further fuel sectarian war,” she said, referring to the Islamic State of Iraq and the Levant, also known as the Islamic State of Iraq and Syria.

News was slow to circulate in Iraq, however, since the government last week blocked social network sites, including YouTube, Twitter and Facebook.

An Iraqi military intelligence official confirmed the military was aware of the reported executions in Salahuddin province, which includes the key city of Tikrit, but he did not know how many there were. He spoke on the condition of anonymity in line with his agency’s rules.

Col. Suhail al-Samaraie, head of the Awakening Council in Samarra, a pro-government Sunni grouping, also confirmed officials in Salahuddin were aware of large-scale executions having taken place last week, but he did not know how many.

“They are targeting anyone working with the government side, anyplace, anywhere,” he said. He said the insurgents were targeting anyone with a government affiliation, whether Sunni or Shiite.

One of those executed by the insurgents was a police colonel named Ibrahim al-Jabouri, a Sunni official in charge of the criminal investigation division in Tikrit, according to Samaraie.

A local journalist familiar with the Iraqi military in Salahuddin province said the 4th Iraqi Army Division had collapsed as the insurgents advanced last week, and 4,000 soldiers were believed to have been captured. Local reports said many of the victims were Sunnis as well as Shiites, he added.

A New York Times employee in Tikrit said by telephone that residents spoke of seeing hundreds of prisoners captured when they tried to flee Camp Speicher, a former U.S. military base and airfield on the edge of Tikrit that was turned into an Iraqi training center. Those who were Sunnis were given civilian clothes and sent home; the Shiites were taken to the grounds of Saddam Hussein’s old palace in Tikrit, where they were said to be executed, their bodies dumped in the Tigris River, which runs by the palace compound.

The Islamic State of Iraq and the Levant photographs appeared to have been taken at that location. However, the Times employee said he had not spoken to any witnesses who claimed to have seen the executions or bodies.

The still photographs uploaded on the Islamic State of Iraq and the Levant Twitter feed were bloody and gruesome, showing the insurgents, many wearing black masks, lining up at the edges of what looked like hastily dug mass graves and apparently firing their weapons into groups of young men who were bound and packed closely together in large groups.

The photographs showed at least five massacre sites, with the victims lying in shallow mass graves with their hands tied behind their backs. The number of victims could be seen in any of the pictures numbered between 20 and 60 in each of the sites, although it was not clear whether the photographs showed the entire graves. Some appeared to be long ditches.

The photographs showed the executioners flying the Islamic State of Iraq and the Levant black flag, with captions such as “the filthy Shiites are killed in the hundreds,” “The liquidation of the Shiites who ran away from their military bases,” and “This is the destiny of Maliki’s Shiites,” referring to Prime Minister Nouri al-Maliki.

Many of the captions were viciously mocking toward the purported victims. In one photograph, showing 25 young men walking toward an apparent execution site, where armed, masked men awaited, the caption read: “Look at them walking to death on their own feet.”

And another showed a couple hundred prisoners, all of whom had been made to stand, bent over from the waist with their hands clasped behind their backs, as armed men guarded them. All were in civilian clothes, and the caption claimed they had jettisoned their uniforms. “They were lions in uniform, and now they are just ostriches,” it read.

Other photographs showed prisoners, mostly young men, stuffed in large numbers in dump trucks and pickup trucks. They appeared extremely frightened.

A senior Iraqi government official, speaking on the condition of anonymity because he was not authorized to make press statements, said news of the executions was slow to circulate because Twitter had been blocked.

“I don’t doubt they are real, but 1,700 is a big number,” he said. “We are trying to control the reaction. They are trying to bring back the 2005 to 2006 days.” Sunni and Shiite militias engaged in a wave of tit-for-tat killings of civilians during that period, killing tens of thousands.

Al-Sistani’s statement late Saturday came only one day after his office had said it was the duty of every Iraqi to take up arms to support the government, which greatly accelerated the formation of volunteer groups, supplementing Shiite militias and planning to support the Iraqi army.
Adamant that his words Friday not be taken as a starting bell for a repeat of that period of bloody sectarian fighting or potentially something even more brutal, al-Sistani used plain language to make clear that he was opposed to and would condemn any sectarian behavior.

All citizens need “to steer clear from sectarian and untamed nationalistic discourse that is of detriment to Iraq’s national unity,” he said.

Both the Iraqi military and the informal Shiite militias already appear to have embraced each other, and in at least some cases, militia commanders are already working inside Iraqi army bases, blurring the line between the government troops and informal squads of gunmen - some of whom may wear government uniforms but are not army soldiers.

Such groupings in 2005 to 2007 were responsible for much of the sectarian bloodletting, when as many as 1,000 civilians were being killed every week.